Naturally occurring acidic waters and acidic wastewaters from industrial processes are found in many geographical areas of the world. Conventional treatment methods commonly employed for treatment of such waters involve neutralization with alkali, to raise the pH, so that the water can be discharged or beneficially utilized. However, such methods are not always desirable, or even feasible in some instances, since such methods can add significant amounts of dissolved solids to the water. And, the cost of the necessary chemicals, and particularly the alkali, can be quite high.
If the treated water is to be utilized for potable applications, one commonly encountered standard which must be met is a World Health Organization criterion that potable water contain no more than 500 milligrams per liter of dissolved solids, and no more than 250 mg/l each of sulfate ion or chloride ion. However, criteria for reuse of water in most industrial applications are far stricter. Consequently, the common “straight neutralization” treatment process is not an acceptable option in a large number of water treatment applications.
In industrial applications, treatment/reclamation of acidic waters is most often presently based on ion-exchange or on reverse osmosis (RO) systems. Depending upon factors such as the level of hardness (polyvalent cations), total organic carbon (TOC), and other contaminants present in the water, anion-exchange can be used for treatment of such acidic feedwaters for the reduction/removal of acidity. Further, the addition of a cation exchange step before or after the anion exchange step can indeed produce water that is almost completely demineralized. For this process, a weak base, an intermediate base, or a strong base anion exchange resin is employed, either singularly or in combination.
The major advantages of such prior art ion exchange treatment methods include the following:
(1) In industry, the method is considered “passive”, meaning that the process is not sensitive to changes in the influent characteristics.
(2) Compared to conventional reverse osmosis, the method has lower capital cost.
The major disadvantages of such prior art ion exchange treatment methods include the following:
(1) The quality, type (e.g., sodium based), and quantity of alkali needed (for regeneration of the IX resin) are actually higher and/or more restrictive than that required for straight neutralization, so the cost of the necessary chemicals is quite high.
(2) A very substantial volume of anion exchange resin is necessary; such resin is generally quite expensive. Thus, the initial and replacement cost of ion exchange resin in such systems is quite high compared to a membrane based treatment system.
(3) Depending upon the specific variety of ion exchange resin utilized, fouling by total organic carbon (TOC) can be quite high. Unfortunately, fouled anion resin can be difficult and expensive to clean. And, non-ioniizable TOC components, such as IPA (iso-propyl alcohol) are not removed. Further, TOC components that are cationic in nature are not removed, either. Typically, removal of TOC, or at least significant reduction of TOC, is often an important requirement in a number of industrial applications where reuse of treated waters is desired.
In conventional membrane based systems that are used for treatment of acidic waste waters or of naturally acidic waters, the pH of the RO/NF feed is commonly adjusted by addition of alkali. Thus, such conventional RO/NF systems operate at, or reasonably close to, neutral pH conditions. With certain exceptions, conventional RO/NF systems are operated under such pH conditions in order to ensure that the RO/NF membranes are not damaged due to very high or to very low pH conditions. More fundamentally, for many commonly encountered membrane materials, the overall solute rejection across the membrane is typically highest at a pH of approximately 8. Thus, the conventional wisdom in the water treatment industry is to avoid operation of RO/NF membranes at low pH conditions.
Yet, some of the basic RO/NF process characteristics point to some particular potential advantages, when compared to ion exchange systems. For example:    (1) RO/NF will simultaneously remove cationic as well as anionic species.    (2) RO/NF will, in general, remove a larger percentage of the TOC present before fouling of the media or membrane becomes a major concern. For instance, RO is capable of removing about 80%, or sometimes more, of non-ionizable species, such as IPA.    (3) The capital, as well as the operating costs of RO/NF systems, unlike those of ion-exchange systems, are not particularly sensitive to the influent water chemistry characteristics.
Nonetheless, the conventional RO/NF systems known to me for treatment of such acidic waters, whether for wastewaters or for naturally occurring waters, still exhibit major shortcomings. Such deficiencies include:    (1) The quantity and cost of alkali needed to neutralize the RO feed remain comparable to mere neutralization, alone. Consequently, overall treatment costs are high, since RO system capital and operating costs must be added to the costs of neutralization.    (2) The combination of pH neutralization followed by RO is fundamentally inefficient, since the total dissolved solids content is first increased by the pH neutralization step, but then the total dissolved solids content is decreased by the RO/NF step.    (3) RO/NF systems are quite susceptible to biofouling and/or particulate and/or organic fouling when they are operated at neutral or near neutral pH conditions. Unfortunately, however, the commonly utilized thin film composite membranes do not tolerate oxidizing biocides, such as chlorine. Consequently, control of biofouling is problematic, especially for treating waters containing organic contaminants.
Thus, a continuing demand exists for a simple, efficient and inexpensive process which can reliably treat acidic waters, whether naturally occurring or a wastewater from another process. It would be desirable to provide water of a desired purity, in equipment that requires a minimum of maintenance. In particular, it would be desirable to improve efficiency of feed water usage, and lower both operating costs and capital costs for water treatment systems, as is required in various industries, such as semiconductors, chemical production, mining, pharmaceuticals, biotechnology, and electric power plants.
Clearly, if a new water treatment process were developed and made available that combines the benefits of both conventional RO/NF membrane treatment and of ion exchange processes, particularly for the treatment of naturally occurring acidic waters as well as industrial waste waters, it would be of significant benefit. Additionally, such a process would be even more attractive if it were immune to the most vexing problems associated with either of reverse osmosis/nanofiltration or of ion exchange. In summary, an economically important new acidic water treatment process would necessarily offer some (if not most) of the benefits of both reverse osmosis and of ion exchange. At the same time, any such new process must be capable of effectively coping with the problems which beset the reverse osmosis/nanofiltration process or the ion exchange process.